Colloidal compositions for electroless deposition stabilized by thiourea

ABSTRACT

Metallic surfaces are imparted to non-conductors or dielectric substrates by electroless (chemical) plating process comprised of coating the substrates with colloid(s) of non-precious metals and wherein the colloids are prepared in a manner as to impart resistance towards further deterioration.

This application is a division of application Ser. No. 938,890 filedAug. 21, 1978 now U.S. Pat. No. 4,265,942 which is acontinuation-in-part of U.S. Application Ser. No. 672,044 filed Mar. 30,1976 which is a division of application Ser. No. 607,506 filed Aug. 26,1975 now U.S. Pat. No. 3,993,799 which is a continuation-in-part ofapplication Ser. No. 512,224 filed Oct. 4, 1974 now abandoned.

BACKGROUND OF THE INVENTION

In the plating of dielectric substrates by chemical (electroless)plating it is well known that suitable catalytic pretreatment is aprerequisite for effective electroless metal deposition. Such practicesare well known and accepted in the art.

In examining the prior art for catalytic pretreatment it appears thatwhile different procedures have been used, the incorporation of preciousmetals (e.g., palladium containing solutions) was common to allprocedures. One catalytic system of particular interest is the two stepprocess as disclosed in U.S. Pat. No. 3,011,920. In the processdisclosed, a colloidal solution composed of tin(II) and precious metalsalts, generally with hydrochloric acid, is used. The effective catalystis proposed to be a colloid of an elemental precious metal (e.g.,palladium) stabilized by the excess stannous chloride present in themedium. While the system disclosed in U.S. Pat. No. 3,011,920 has beenquite popular in commercial practices, rising costs of precious metal,instabilities due to air oxidation, and miscellaneous productreliability problems have led to the quest for new systems in which theuse of precious metal as well as hydrochloric acid would be completelyeliminated.

In meeting this objective it was found, as described in U.S. Pat. Nos.3,958,048, 3,993,491, 3,993,799, and 4,087,586 that colloidal systemsbased upon non-precious metals could constitute the basis for newcommercial plating processes. More specifically, it was found andreported that colloidal compositions of non-precious metals (preferablyselected from the group of copper, cobalt, iron, nickel and manganese)may be used in the direct replacement of the tin/palladium colloidfollowed by a treatment (which may be optional) in a suitable reducingmedium (activating medium). In the latter treatment a precursor derivedfrom the colloidal dispersion constitutes the catalytic sites useful inthe initiation of plating. In the reducing medium, reduction of theionic portion of the adduct derived from adsorption in the colloidalmedium takes place, or surface activation, which results in activenucleating sites capable of initiation of the electroless process.Alternatively, the second step may encompass the selective dissolutionof a colloid stabilizer(s) thereby exposing the catalytic nucleus of thecolloid, or contacting with a solution comprising soluble compound(s) ofcatalytic metal. Hence, it should be recognized that the step ofactivation which is optional generally is intended to reduce theinduction time for the actual electroless metal build-up or to removeweakly adsorbed colloids, thereby preventing their contaminating theplating bath(s).

The colloidal nucleus may be in a form of a metallic (elemental) state,or compound bearing the metal, or alloy, and mixtures thereof and themetal(s) must be of a catalytic nature in at least one of its possibleoxidation states.

In reviewing the teaching disclosed in the aforementioned issued patentswhich are included herein by reference, it is recognized that many ofthe inherent disadvantages associated with the palladium based catalystsare eliminated. It is further recognized that based upon practices inthis art it is further essential that any catalytic system shouldmaintain its properties especially with storage (e.g., several months)and shipment under conditions of substantial temperature fluctuation. Itis thus highly desirable to have a medium in which good colloidalstability would be maintained, and which at the same time has sufficientcatalytic activity to be used in the plating process. I have observedthat as one increases stability, activity is decreased thereby making itdifficult to meet both requirements in a single preparation step.

For example, I have observed that with successful synthesis of activeplating colloids, there is generally a limited stability (for long termpurposes) due to coagulation which takes place leading to precipitation,with, of course, change in particle size and distribution during thecoagulation process. Also, at times dissolution of the colloidal statemay also take place with time. In addition, I have noted that highlystable colloidal dispersions have shown limited catalytic activity whenused in accordance with U.S. Pat. No. 3,993,799 with a moderateconcentration of reducing medium or activating medium or the omission ofany secondary step. Similar trends were also noted in U.S. Pat. No.3,948,048 on the interrelationship between reactivity and stability. Infact, in U.S. Pat. No. 3,958,048 most of the illustrated examples, whenrepeated, lost their colloidal character and became true solutionswithin 24 hours probably due to the interaction of the colloid with airor more particularly with oxygen. Many times the deterioration of thecolloid is manifested in visible color change(s). Hence, simple visualobservation could determine whether a colloid is deteriorating.

It is thus highly desirable to provide stable colloidal dispersionswhich would maintain their integrity and resist deterioration byprecipitation and/or contact with air. Such colloids may be useful inelectroless plating process, catalysis, or any other processes utilizingcolloids. It is further desirable to obtain dispersions with very fineparticle size distributions. Small size dispersions are particularlyuseful in adsorption processes and catalysis.

While not wishing to be repetitious, the following are included hereinby reference: U.S. Pat. Nos. 3,011,920, 3,993,799, 3,524,754, 3,958,048,3,993,491, 3,993,801, 4,087,586, 4,048,354, British Pat. No. 1,078,439,and copending applications Ser. No. 712,131 now U.S. Pat. Nos.4,136,216, 820,904 now U.S. Pat. Nos. 4,131,699, 833,905 now U.S. Pat.Nos. 4,151,311 and 854,909 now U.S. Pat. No. 4,132,832.

SUMMARY OF THE INVENTION

Novel colloidal compositions useful in electroless plating processes aredisclosed which are particularly stable against further deterioration.

DETAILED DESCRIPTION OF THE INVENTION

The process and composition of the present invention is applicable tothe metallic plating of dielectric substrates by autocatalytic or asmore commonly known, electroless (chemical plating. Such processes arewell known in the art and they produce a wide variety of productsvarying from printed circuitry arrays to decorative plated plasticsparts.

In some of the above applications the colloids are used afterpreparation without any further change; however, in others changes maybe induced after preparation, e.g., change in the oxidation state of aportion of the colloid.

Although there are many methods for the preparation of colloidaldispersion, the use of the precipitation (chemical) method has beenquite popular. In the latter method, the insoluble phase is developed(nucleated) through the interaction of at least two reactants, e.g., ametal compound with a metal reducing agent, or alternately, a solublemetal salt with an alkaline agent. Both reactants should preferably besoluble in a suitable solvent. For general survey of preparatory methodssee B. Jirgensons and M. E. Straumanis, "A Short Textbook of ColloidChemistry," 2nd Edition, The MacMillan Company, New York (1962). Thepresent invention would be illustrated through the preparation ofcolloidal dispersions by precipitation (chemical) method; however, it isclear that the invention is not limited to the preparation methodselected.

The method described in this invention may be applied to any of severalcolloidal compositions of non-precious metals; the metal may be part ofa compound, alloy, or in the metallic state, as well as combinationsthereof. Preferred metals are those which are catalytic for electrolessmetal deposition. Such metals are well known in the art and they arerecited in U.S. Pat. Nos. 3,011,920, 3,993,799 and many others.

The term "colloid stabilizer" as used herein is intended to encompasssubstances which alter the characteristics of the colloid so as toprevent, delay, or minimize their coagulation and precipitation.Stabilizers may be organic or inorganic substances and mixtures thereof.It is believed that these stabilizers are adsorbed onto the surface ofthe colloid thereby altering the surface charge and hence theirstability. Stabilizers contemplated by the present process andcomposition may include but are not limited to secondary colloids,protein, gelatin, agar agar, gum arabic, surfactants, sugars andpolyalcohols (glycerol), and miscellaneous chemicals derived from wood,e.g., lignin, hemicellulose. It is noted that gelatin is a form ofprotein. In the general sense it is recognized that stabilizers areinherently adsorbed onto the nucleus of the colloid or participatewithin the double-layer structure of the colloid. Moreover, it isrecognized that in the colloidal dispersion preferably at least onecolloid stabilizer must be present.

The term "surfactant" (or surface active agent) as used herein generallyrefers to substances which are capable of lowering the surface tensionof a liquid or the interfacial tension between two liquids. All usefulsurfactants possess the common feature of a water-soluble (hydrophilic)group attached to an organic (hydrophobic) chain. Surfactants as usedherein are also intended to encompass detergents, dispersants, andemulsifying agents regardless of whether or not they lower the surfacetension of a liquid (e.g., water).

The term "primary metal" as used herein generally refers to metalselected from the group consisting of copper, nickel, cobalt and ironand mixtures thereof. These metals exist in the colloidal catalyticcomposition as a reaction product which may be in the elemental state,compound, or alloy, and mixtures thereof. For a source of the primarymetals soluble or insoluble compounds bearing such metals may be used.

The term "secondary metal" as used herein generally refers to metalsselected from the group of metals selected from Groups IIIA, IVA, and VAof the Periodic Table of the Elements and preferably selected from thegroup consisting of antimony, lead, thallium, tin and chromium, andmixtures thereof. These metals may exist in the colloidal catalyticcomposition as a reaction product along with any of the primary metalsand further they may be in the elemental state, compound, or alloy, andmixtures thereof. For a source of the secondary metals soluble orinsoluble compounds bearing said metals may be used. It is alsorecognized that other metals such as zinc, cadmium, indium, and bismuthare similar in their chemical properties to the above secondary metals.Hence, the substitution of these metals falls within the spirit of thepresent invention. In preparing the colloidal composition and theutilization of the secondary metal(s), it is preferable to use thesemetals in compounds in which they are in the lowest oxidation state,e.g., Cr in +3 oxidation state rather than in +6 oxidation state. Whilegenerally these secondary metal(s) are not as active as the primarymetal(s), their incorporation generally results in maintaining theintegrity of the colloids and in particular averting the deteriorationof the colloids. While I do not wish to be bound by theory, it isbelieved that the secondary metal(s) react chemically with the primarymetal(s) leading to a reaction product(s) which provides with theimproved results against deterioration. The reaction product(s) may bean alloy(s) or compound(s) of these metals. It is also noted that manyof the secondary metals, when used without reacting with the primarymetal(s), or added post colloid nucleation, act as inhibitors asdemonstrated in Ser. No. 833,905. These metal(s) may exist in a wideconcentration range relative to the primary metal(s).

The term "precipitating agent" as used herein refers to chemicalsubstances which upon their interaction with compounds bearing thecatalytic metal(s) (primary and/or secondary metals) lead to theformation of an insoluble (colloidal) phase in the solvent (e.g.,water). Any of several chemical substances may be used ranging fromreducing agents (e.g., hydrides and its derivatives, hypophosphorus acidand its derivatives, hydrazine, formaldehyde, tannic acid, dithionate,sulfites, etc.) to hydroxides, sulfides, chromates, phosphate andothers. Also, active metals may be used as possible reducing agent,e.g., zinc with copper ions. Depending upon the nature of theprecipitating agent any of several types of isoluble phases may beformed. The use of precipitating agents is well documented and isfurther demonstrated in the references included by reference in thepresent application. In the formation of the colloidal state, many timesthermal energy must be supplied as to overcome the activation energyrequired for nucleation of the isoluble phase.

Generally speaking it is noted that non-conductor substrates may beclassified as vitreous type and organic type, depending upon theresidual charge on the surface. The former generally constitutematerials which are ceramic, glass, and the like, which generally arenegative surfaces due to oxygen exposed at the interface. By contrast,the second group generally constitute organic materials which ingeneral, and especially upon etching of such substrates, are positivelycharged. Hence, for effective adsorption it should be recognized that itis generally preferable to use colloids which are charged in theopposite charge of the substrate surface which is to be metallized,though intermediate "layers" may be used onto the substrate which alterits effective charge.

The following examples are provided to illustrate the findings of thepresent invention. These examples are not to be taken as in limitationof the invention, but it should be recognized that the inventionencompasses various combinations thereof and the concentrations may bevaried.

EXAMPLE 1

An alumina ceramic substrate was immersed into a colloidal compositioncomprising the reaction product of the following components. Thereaction was carried forth at about 50° C.

    ______________________________________                                        CuCl.sub.2              0.04M                                                 Sn(BF.sub.4).sub.2      0.032M                                                Gelatin                 5.0g/l                                                NaBH.sub.4              0.05M                                                 NaOH                    0.5M                                                  ______________________________________                                    

The substrate was immersed for several minutes and thereafter waterrinsed, and directly immersed into a commercial electroless copper(Enplate 404) operating at room temperature. Spontaneous plating ofcopper was noted.

In the course of evaluating compositions of tin/copper it wassurprisingly and unexpectedly found that:

1. The product exhibits a greater stability when exposed to air incomparison to copper alone.

2. Similar improvement of the copper colloid was achieved by thecombination of two distinct colloidal compositions: the first was copperalone and the second was of tin alone. Hence, both modes of preparationfall into the spirit of the present invention.

3. Examination of tin/copper colloidal product by electron microscopydiffraction and transmission modes appears to yield new interactionproduct(s), probably an alloy of these metals. Specifically, it wasnoted that the diffraction patterns showed the disappearance of certaind-spacings as well as the formation of new lines. Also noted wereparticles of about 50 A in size whereas the particles of the tin colloidalone were much greater in size.

4. The concentration for tin relative to copper can be used over a widerange.

Substitution of nickel or iron or cobalt for copper or in addition tocopper is self evident.

EXAMPLE 2

A colloidal composition was prepared comprising the reaction product ofthe admixture comprising

    ______________________________________                                        CuCl.sub.2              0.04M                                                 TlNO                    0.002M                                                Arabic Gum              11.8g/l                                               NaBH.sub.4              0.04M                                                 NaOH                    0.5M                                                  ______________________________________                                    

The resulting pH was 12 and the nucleation reaction was carried outabove room temperature. Comparison of this resulting colloidal productto copper alone showed a greater stability towards exposure to air.

EXAMPLE 3

A colloidal composition comprising of copper and tin was prepared by theadmixing of:

    ______________________________________                                        CuCl.sub.2 . 2H.sub.2 O 10.6g/l                                               Sodium citrate          25g/l                                                 SnCl.sub.2 . 2H.sub.2 O 19.2g/l                                               ______________________________________                                    

Prior to the addition of the tin, the solution (at pH 14) was heated toabout 100° C. and final pH adjustment of the product was made to about12.5. The formation of a brown-gray colloid was noted.

Immersion of an etched ABS substrate followed by electroless coppershowed that plating has taken place. The preparation of the abovecolloid was made based upon Weiser, Vol. I, "Inorganic ColloidChemistry", p. 137, John Wiley & Sons (1933).

EXAMPLE 4

A colloidal composition comprising the admixture of the followingchemicals was nucleated above room temperature. The nickel was addedpost copper colloid nucleation and the final pH was adjusted to 8.0.

    ______________________________________                                        CuCl.sub.2              0.04M                                                 NiCl.sub.2              0.01M                                                 NaBH.sub.4              0.039M                                                NaOH                    0.196M                                                (NH.sub.2).sub.2 CS     0.0067M                                               Orazan-S                12.0g/l                                               ______________________________________                                    

This product showed a superior stability to deterioration in comparisonto the same product in the absence of added (NH₂)₂ CS. While I do notwish to be bound by theory, it is believed that the addition of thiourearesults in adsorption onto the colloid and provides protection againstfurther deterioration. Homologs of thiourea were also used, e.g., (NH₂)₂CO and (NH₂)₂ CSe. However, their effectiveness was not as good as thethiourea. It is further realized that it is the thio group ##STR1## thatprovides the present effect and thus substituting other thio containingcompounds falls within the spirit of this invention. R₁ and R₂ may bevarious groups from alkyl, amines, aromatics, hydrogen, halogen, andmixtures thereof.

EXAMPLE 5

A colloidal composition comprising the reaction product resulting fromthe admixture of the following reactants was prepared. Nucleation tookplace above room temperature. Final pH was about 12. The resultingcolloidal composition showed a greater visual stability in comparison tothe same without the added antimony. Hence, antimony in combination withthe copper primary metal is a useful combination.

    ______________________________________                                        CuCl.sub.2              0.04M                                                 SbCl.sub.3              26.8g/l                                               Arabic Gum              8.8g/l                                                NaBH.sub.4              0.049M                                                NaOH                    0.49M                                                 ______________________________________                                    

EXAMPLE 6

A colloidal composition similar to above was prepared, except that leadwas used instead of the antimony trichloride. Specifically, the lead wasPb (from fluoroborate solution) at 0.04 M. Final pH about 12 and greaterstability were noted. Substitution of equimolar Cr⁺³ for the lead alsoshowed a tendency for the formation of a more stable product(s) relativeto copper alone.

While the above examples are provided to illustrate the novel aspects ofthe present invention one skilled in the art should recognize that manyother chemicals and means may be used to achieve the useful results ofthis invention. Such adaptations and modifications fall within thespirit of the present invention. Furthermore, it is possible that thecolloidal composition may be prepared by the suspension of an availabledry powder or semi-dried powder in a suitable solvent; such approach incombination with the composition disclosed falls within the spirit ofthis invention. In using the present invention both soluble andinsoluble compounds may be used as starting chemicals for the colloidpreparation though soluble compounds are preferred. Moreover, any ofseveral oxidation states may be applied (e.g., Cu(I) and Cu(II)).

It is further noted as recognized in U.S. Pat. Nos. 3,993,491 and3,993,799 that compatible electroless plating compositions should beused for achieving best results.

It is further recognized that in the event that the insoluble phase(colloid) is formed with the assistance of a precipitating agent, thesolvent to carry forth the reaction may be of aqueous or non-aqueoustype. Moreover, contemplating the present invention the colloidalcomposition may be dispersed in aqueous or non-aqueous solvents.

What I claim is:
 1. A method for improving the stability of a colloidaldispersion comprising a non-precious metal and a solvent wherein saidmetal is catalytic for electroless metal initiation comprising the stepof incorporating a compound selected from the group consisting of##STR2## (NH₂)₂ C═Se, and (NH₂)₂ C═O and mixtures thereof and wherein R₁and R₂ are selected from the group consisting of alkyl, amines,aromatics, hydrogen and halogen into said colloidal dispersion therebyimproving the stability for said colloidal dispersion.
 2. The methodaccording to claim 1 wherein said catalytic metal is copper.
 3. Themethod according to claim 1 wherein said compound is thiourea.
 4. Themethod according to claim 1 wherein said catalytic metal is copper andwherein said compound is thiourea.
 5. The method according to claim 1wherein said catalytic metal is part of an elemental state.
 6. Themethod according to claim 1 wherein said catalytic metal is part of analloy.
 7. The method according to claim 1 wherein said compound is addedsubsequent to the nucleation of the colloid.
 8. The method according toclaim 1 wherein said electroless metal deposition is of copper.
 9. Themethod according to claim 1 wherein said colloidal dispersion is aqueousbased.
 10. The method according to claim 1 wherein said colloidaldispersion further contains a colloidal stabilizer.
 11. A process forthe electroless metallization of a non-conductor substrate comprisingthe steps of:(1) contacting said substrate with a colloidal dispersioncomprising a non-precious metal catalytic for electroless metalinitiation and a compound selected from the group consisting of ##STR3##(NH₂)₂ C═Se and (NH₂)₂ C═O and mixture(s) thereof and wherein R₁ and R₂are selected from the group consisting of alkyl, amines, aromatics,hydrogen, and halogen, and (2) contacting the treated substrate with acompatible electroless plating composition.
 12. The process according toclaim 10 wherein said non-precious metal is copper.
 13. The processaccording to claim 10 wherein said compound is thiourea.
 14. The processaccording to claim 10 wherein said non-precious metal is copper andwherein said compound is thiourea.
 15. The process according to claim 10wherein said electroless plating is copper.
 16. The process according toclaim 10 wherein said substrate is for a printed circuitry array. 17.The process according to claim 10 wherein said substrate containsthrough-holes.
 18. The process according to claim 10 further containingthe step of activation, said activation taking place subsequent to thecontacting of the substrate with the colloidal dispersion, however priorto the step of electroless plating.